Broadly stated, the objectives of the research presented here are to elucidate neuroendocrine mechanisms underlying reproductive behavior and to analyze the communicative role of ultrasonic vocalizations in the social integration of reproduction. Research proposed on neuroendocrine factors will be concerned primarily with sites and mechanisms of steroid hormone action in the regulation of sexual behavior. To accomplish this goal hormonal and other agents will be implanted directly at the sites of populations of target cells in the central nervous system in male anf female rats. Research on ultrasonic vocalizations will be focused on the nature of the message conveyed by 50 kHz vocalizations of males and females and by the 22 kHz postejaculatory vocalization of the male. Techniques including surgical devocalization and playback of recorded vocalizations will provide experimental control over these signals. Research proposed here should extend our understanding of funamental neuroendocrine mechanisms underlying reproductive processes in mammals in general (including humans). In addition, expansion of our understanding of the dynamics of social communication in reproductive activity of rats will make this species an even more valuable model system for the study of reproductive processes in general.